1. Field of the Invention
This invention relates to high range, wide-angle zoom lenses of compact form suited to leaf shutter cameras or video cameras and, more particularly, to zoom lenses for photography which are excellent in portability as the angle of field is increased while shortening the total length of the entire system (or the distance from the front vertex to the image plane).
2. Description of the Related Art
Recently, in the field of art of leaf shutter cameras, or video cameras, the trend of improving the compact form of the camera housing has prevailed. Along with this came a growing demand for the zoom lens of the compact form which is improved in respect to the total length of the entire system.
Particularly for the leaf shutter camera, owing to the achievement of a great advance of peripheral techniques such as the electric circuit of driving the zoom actuator and due to other reasons, the minimization of the size of the camera is ever improving. The photographic lens which such a camera will be equipped with, is required not only to have a high zoom ratio, but also to get a compact form.
In the prior art, the zoom lenses for the leaf shutter cameras are made up using two components of positive and negative refractive powers. That is to say, the so-called 2-component type of zoom lenses is the mainstream type. This 2-component zoom lens has a simple configuration and is quick and easy to operate, as the moving mechanism is simple in the structure of construction. Therefore, it is amenable to the technique of even more reducing the size of the camera. Another merit is that the production cost is comparatively low.
However, the function of varying the focal length has to be laid on only one component. The zoom ratio is, therefore, limited to the order of 1.6 to 2 or thereabout. The much desired increase of the zoom ratio invites a rapid increase of the bulk and size of the entire lens system. At the same time it becomes difficult to maintain high optical performance.
With the 2-component zoom lens as basis concepts, the first component may be divided to two parts whose refractive powers are both positive, thus making up the entire system from three components of positive, positive and negative refractive powers, so that a great increase of the zoom ratio is aimed at. Such a 3-component zoom lens is proposed in, for example, Japanese Laid-Open Patent Applications Nos. Hei 3-282409, Hei 4-37810 and Hei 4-76511.
However, if the artisan attempts to employ this configuration to realize as wide angle a zoom lens as, for example, 35.degree. or more in semi-angle, it will result that the entrance pupil varies in position to a large extent with zooming. Because of this, the much desired increase of the zoom ratio cannot be achieved without causing rapid increase of the difficulty of suppressing the variation of aberrations with zooming.
Besides these, Japanese Laid-Open Patent Applications Nos. Hei 2-72316 and Hei 3-249614, for example, have made the previous proposal for using multi-component technique in increasing the semi-angle at the wide-angle end to about 38.degree. and the zoom ratio to about 3.5 at once.
However, these zoom lenses have become larger in both the diameter of the front members and the total length of the entire system. So, they are not always satisfactory enough as a photographic lens for use in the compact camera.
Particularly in application to the camera of the type using the external finder, a problem arises in that when zooming to the wide-angle end, the lens barrel shuts off in part the finder field of view. As a result, another problem even arises in that serious limitations must be put on the finder arrangement and the layout of the camera.
In the meantime, the conventional examples of zoom lenses having four components of minus-plus-plus-minus power arrangement are disclosed in U.S. Pat. Nos. 4,787,718, 4,756,609, 5,111,338, 5,270,865 and 5,274,504 and Japanese Laid-Open Patent Applications Nos. Hei 4-15610 and Hei 4-237009. Any of these moves the first and second components in such a way that their separation decreases when zooming from the wide-angle end to the telephoto end.
With this, the U.S. Pat. No. 4,787,718 takes zoom ratio at 2 or thereabout. The U.S. Pat. Nos. 4,756,609, 5,270,865 and 5,274,504 and Japanese Laid-Open Patent Application No. Hei 4-15610, though having given a zoom ratio of 3 or so to their lens systems, are insufficient in increasing the angle of field.
In the U.S. Pat. No. 5,111,338 and Japanese Laid-Open Patent Application No. Hei 4-237009, the angular field and the zoom ratio have been made wide enough and high enough, but the total number of constituent lenses is very large. Moreover, the front members have got as large a diameter as is not suited to actual use.
In general, the zoom lenses have a feature that the stronger the refractive power of each of the components is made, the shorter each of the required movements of the zoom components for the equivalent range becomes, thus making it possible to simultaneously fulfill the requirements of increasing the zoom ratio and of shortening the total length of the entire system. However, the use of a simple technique by increasing the refractive power of each of the components leads to an increase in the range of variation of aberrations with zooming. Particularly for a case that the zoom ratio and the angle of field are to increase simultaneously, the problem of maintaining good stability of high optical performance throughout the extended zooming range becomes very difficult to solve.